The proposed experiments are designed to use two approaches, acute neurophysiological techniques and psychophysical methods, to study the involvement of the central system in pain perception and its regulation. The neurophysiological studies will investigate the action of four descending systems, the raphe-spinal system, the reticulospinal system, and descending projections from the sensorimotor cortex and the orbital cortex, on the cells of origin of three ascending projections, the spinothalamic tract, the spinocervical tract and the dorsal spinocerebellar tract. Emphasis will be placed on using conventional histogram techniques, cross correlation analysis, and Wiener kernel analysis to determine how the descending systems modify the responses of these neurons to noxious and non-noxious inputs applied to the center and the periphery of the receptive fields. The psychophysical experiments will examine the role of distraction, motor activity and suggestion on the perception of noxious and non-noxious thermal stimuli using a paradigm based on signal detection theory. Other psychophysical studies will evaluate the interaction of transcutaneous electrical stimulation and aspirin. Additional experiments will study the changes in the perception of acute thermal stimuli resulting from any long-term modifications in sensory function associated with the chronic pain state.